Osteoporosis describes a group of diseases which arise from diverse etiologies, but which are characterized by the net loss of bone mass per unit volume. A consequence of this loss of bone mass is the failure of the skeletal frame to provide adequate structural support for the body, resulting in bone fracture. One of the most common types of osteoporosis occurs in women shortly after menopause. Most women lose between 20-60% of the bone mass in the trabecular compartment of the bone within 3-6 years after the cessation of menses. This rapid loss of bone loss is generally associated with an increase of both bone resorption and formation. The resorptive cycle is more dominant, however; and the result is a net loss of bone mass.
Thus, osteoporosis is a common and serious disease among post-menopausal women. An estimated 25 million women in the United States alone are afflicted with this disease. The results of this disease are both personally and economically harmful.
Large economic losses are due to its chronic nature and the need for extensive and long term support (hospitalization and nursing home care) from the disease sequellae. The losses are especially great in more elderly patients. Additionally, although osteoporosis is not generally considered a life threatening condition, there is a 20-30% mortality rate related to hip fractures in elderly women. A large percentage of this mortality rate can be directly associated with post-menopausal osteoporosis.
The tissue in the bone most vulnerable to the effects of post-menopausal osteoporosis is the trabecular bone. This tissue is often referred to as spongy or cancellous bone and is particularly concentrated near the ends of the bone, near the joints and in the vertebrae of the spine. Trabecular tissue is characterized by small osteoid structures which inter-connect with each other and with the more solid and dense cortical tissue that makes up the outer surface and central shaft of the bone. This criss-cross network of trabeculae gives lateral support to the outer cortical structure and is critical to the bio-mechanical strength of the overall structure. It is primarily the net resorption and loss of the trabeculae which leads to the failure and fracture of bone in post-menopausal osteoporosis. In light of the loss of the trabeculae in post-menopausal women, it is not surprising that the most common fractures are those associated with bones which are highly dependent on trabecular support, e.g., the vertebrae, the neck of the weight bearing bones (femur) and the forearm. Indeed, hip fracture, collies fractures, and vertebral crush fractures are hall-marks of post-menopausal osteoporosis.
A very important concept in the treatment and study of post-menopausal osteoporosis is the concept of fracture threshold. The fracture threshold is the point at which the bone density (therefore, the bone strength) decreases to a value where there is a high probability of bone fracture. This point is not a particular value for all women but rather a relative number for an individual and is dependent on a number of factors such as weight, life-style, or other risks which might contribute to the possibility of bone fracture.
In general, most pre-menopausal women have bone densities above the fracture threshold, and there is a low probability that a fracture will occur. A woman's pre-menopausal bone density and the rate of bone loss after menopause will determine when, or if, she will cross the threshold and be at risk for fracture. For women who present with fractures due to osteoporosis, ideal therapy would be to increase bone density (strength) to a value above the fracture threshold. Alternatively, for women whose bone density is still above the threshold, it would be advantageous to keep them above it.
Today, the only available effective treatment for post-menopausal osteoporosis is hormone replacement therapy, specifically estrogen replacement because post-menopausal women are estrogen deficient. The mechanism of action of estrogen in the treatment of osteoporosis is not well understood; however, it is generally agreed that it inhibits bone resorption. The net effect of the estrogen replacement therapy (ERT) is to keep the woman's bone density at the level at which therapy was initiated, i.e., it maintains bone density. If a woman is above the fracture threshold when (ERT) is initiated, and if ERT is maintained, she will remain above the threshold and be at low risk for fracture. This fact would argue for the placement of women on ERT at or soon after the cessation of menses.
For women whose bone density has already fallen below the fracture threshold, however, ERT will only maintain bone density at the level at which they began therapy. Thus, these women will remain below the threshold and will be at further risk for fracture. ERT is still advisable for these women because it will keep a bad situation from getting worse. It would clearly be advantageous, however, to have a therapy which would boost bone density above the fracture threshold to more normal levels and then maintain it. Currently, there are no effective approved therapies which demonstrate an ability to increase bone density to such a level.
As noted, ERT is now the only effective approved treatment for post-menopausal osteoporosis. In those women who do not have a uterus, estrogen (usually given as a conjugated form of estrone) can be given by itself. In most post-menopausal women who have a uterus, however, unopposed estrogen increases the risk of endometrial cancer. Thus, a progestin is often also administered, either as a combination or in cyclical therapy, to reduce that risk.
"Antiestrogen" is a term that "has been rather broadly applied to several different types of compounds that inhibit or modify the action of estrogen. Progestins and androgens have been described as antiestrogenic . . ." (Goodman and Gilman, The Pharmacological Basis of Therapeutics, 6th Ed., p 1431.) In addition, certain synthetic compounds, such as tamoxifene, clomiphene, raloxifene and nafoxidine, are called antiestrogens and have been shown both experimentally and clinically to block some of the effects of estrogen. The synthetic "antiestrogens" were principally developed for the treatment of estrogen-dependent breast carcinoma. These compounds are classical mixed agonist/antagonists which demonstrate some estrogenic activity. For example, tamoxifene, the most widely used antiestrogen, has been shown to have estrogenic effects in humans.
Recently, the "antiestrogen" group typified by raloxifene (formerly called keoxifene) has been shown to inhibit bone loss in animal experiments, indicating that these compounds should be useful in the treatment of post-menopausal osteoporosis. (See our copending application entitled, METHODS FOR INHIBITING BONE LOSS, Ser. No. 07/920,933, filed Jul. 28, 1992).
We have now discovered that administering a raloxifene-type compound of formula I together with certain progestins can return bone density to nearly normal levels, an effect which neither agent is capable of doing alone.